This invention is generally directed to toner and developer compositions, and, more specifically, the present invention is directed to toner compositions and imaging processes thereof. In embodiments, there are provided in accordance with the present invention toner compositions containing copolymer resins or copolymer resin blends which are monomodal or possess a nearly monodisperse molecular weight distribution characteristic. In another embodiment, toner resins of the instant invention provide an optimum combination of mechanical and rheological properties, low melt viscosity and melt fluidity, low fusing temperatures and broad fusing latitudes. In another embodiment, there are provided in accordance with the present invention, imaging processes with toner compositions having fused toner images with gloss characteristics, measured by a gloss meter, that are determined by the molecular weight properties of the resin copolymer and copolymer resin blends selected.
Preferred low melt xerographic toners compositions of the instant invention are formulated with monomodal resins or blends thereof. Monomodal resins of the instant invention have a single peak, as determined using gel permeation chromatography analysis, and have a polydispersity or ratio of weight average molecular weight M.sub.w and number average molecular weight M.sub.n of between 1 and 3 and preferably between 1 and 2. Resins which are monomodal and monodisperse or substantially monodisperse provide optimum combinations of the aforementioned properties and afford a simple and convenient means by which to control the gloss characteristics of fused toner images. The ability to control the gloss characteristics of fused toner images is important for achieving, for example, high quality gloss characteristics in xerographic pictorial color applications and high quality matte finish characteristics in black or monochrome applications. Moreover, high projection efficiency with transparencies requires smooth, high gloss images to reduce scattering of incident light on image surfaces. The resins of the present invention allow the formation of matte text images and glossy pictorial images made with different toners when fused under the same fusing temperature conditions.
One presumption in the field of xerographic image fusing according to U.S. Pat. Nos. 4,973,538, 4,795,689, 4,386,147, 4,499,168, 4,910,114, 4,968,574, 5,001,031, 4,917,984, and 5,057,392 is that toner compositions having desirable broad fusing latitudes are obtained with polymers or copolymers and blends thereof having broad molecular weight distributions or large polydispersity values. A seemingly plausible rationale for this presumption is that high molecular weight macromolecules with high melt viscosities will blend with and tend to rheologically reinforce lower molecular weight macromolecules having lower melt viscosities. The presumed reinforced blend thereby prevents offsetting of the lower molecular weight, lower melt viscosity macromolecule component of toner images from a receiver sheet to a fuser roll in a conventional xerographic thermal fusing process step. This aforementioned presumption has further led to the deliberate preparation of toner polymers having broad molecular weight distributions and to toner developer materials designs having at least some very high molecular weight polymer component to reinforce lower molecular weight components.
For example, U.S. Pat. Nos. 4,973,538, 4,795,689, 4,386,147, 4,910,114, 4,968,574, 5,001,031, 4,917,984, and 5,057,392, teach a reinforced melt concept wherein a number of multimodal toner polymers are indicated to provide unique broad fusing latitude performance. It is now evident from applying the resin compositions and processes of the present invention that broad molecular weight distributions may not be necessary to obtain broad fusing latitudes and that broad molecular weight distributions can, in instances, actually adversely affect the desired high gloss characteristics of the fused toner images when fused under conventional roll fusing system conditions.
In embodiments, the preparative processes of the present invention comprise preparing a monomodal-monodisperse copolymer toner resin by copolymerizing olefin containing monomers such as styrene and butadiene, for example, in a non-aqueous medium with preferably an anionic polymerization initiator, by cooling between -40.degree. and 0.degree. C. in 25 weight percent tetrahydrofuran and 75 weight percent cyclohexane solvent system for several hours. Monomodal and monodisperse resins are formed, for example, poly(styrene-butadiene) having a molecular weight range from about 5,000 to about 75,000 and a polydispersity (M.sub.w /M.sub.n) of from 1.0 to about 2.0. Adding and dispersing pigment particles and known performance additives in the copolymer resin or a blend of two or more monomodal resins affords toner compositions having the aforementioned advantages. The resins may be processed into toner particles by conventional melt-mixing methods followed by conventional jet mill attrition techniques.
The resulting toners and developer compositions can be selected for known electrophotographic imaging and printing processes, especially dry and liquid development xerographic imaging and printing processes, including color processes, and lithography.
In some xerographic systems wherein process color is a necessity such as pictorial color applications, toners having low fusing temperatures such as from about 100.degree. to about 140.degree. C. are preferable, for example, to avoid paper curling and to maximize gloss properties. Lower fusing temperatures minimize the loss of moisture from paper, thereby reducing or eliminating paper curl. Furthermore, in process color applications and especially in pictorial color applications, high gloss is often necessary, as well as high projection efficiency properties for transparency images.
Numerous processes are known for the preparation of toners, such as, for example, conventional processes wherein a resin is melt kneaded or extruded with a pigment, micronized and pulverized to provide toner particles. Additionally, toners must not aggregate or block during manufacturing, transport or storage periods before use in electrographic systems and must exhibit low fusing temperature properties in order to minimize fuser energy requirements. Accordingly, toner resins exhibit glass transition temperatures of from more than about 50.degree. C. and preferably of from more than about 55.degree. C. to satisfy blocking requirements. This blocking requirement restricts the toner fusing properties from about 135.degree. C. to about 160.degree. C. In process color or pictorial applications, wherein low paper curl is a requirement, low temperature toner fusing properties are desired such as less than about 140.degree. C. and preferably less than 110.degree. C. such that moisture evaporation or removal from paper is minimized or preferably avoided. Toners of the present invention, fuse at relatively lower temperatures such as from about 110.degree. to about 150.degree. C., thereby reducing the energy requirements of the fuser and more importantly resulting in lower moisture driven off from the paper during fusing, hence lowering or minimizing paper curling necessary for pictorial applications. For the toners of the present invention, blocking, fusing, and gloss properties may be controlled by judicious selection of a monomodal resin or a blend of monomodal resins as described herein. Thus, in embodiments of the instant invention are described selection criteria for obtaining: high, intermediate and low gloss fused toner image appearance; broad and narrow toner fusing latitude as measured by crease and gloss properties; and preferred toner blocking temperature properties. The minimum fix temperatures of matte or non-glossy toner images are measured by image crease tests, whereas minimum fix temperatures of glossy pictorial images are measured using a VWR 75.degree. gloss meter.
In general, a crease minimum fix temperature of a toner composition is dictated by the toner glass transition temperature, T.sub.g, wherein lower toner T.sub.g values translate into lower crease minimum fix temperature (MFT).
The crease fusing latitude of a toner is determined by the M.sub.w of the toner resin. The higher the M.sub.w of the resin, the greater the fusing latitude of the toner. The fusing latitude of a toner approaches a maximum plateau when the weight average molecular weight of the toner resin approaches about 45,000. Thus, preferred low melt toners with respect to low crease MFT and broad fusing latitude are those toners made with the highest molecular weight resin materials which allow acceptable toner jetting rates to be maintained. Because toner jetting rates decrease logarithmically with increasing copolymer molecular weight, toner resin designs are practically limited to those resins which jet fast enough to be cost effective, that is, for example, resins with number average molecular weights less than 30,000.
For the resins with broad polydispersities evaluated in the present invention, the fusing behavior of the toner is severely limited by the lowest molecular weight components in the resin composition. Most polymers show a strong T.sub.g to molecular weight dependence in which lower molecular weight polymers have lower T.sub.g values. Consequently, most polymers with broad polydispersities are composed of both high and low T.sub.g components, and the measured Tg represents an average of all the respective T.sub.g values of all resin components. The T.sub.g of the toner resin relates to its blocking temperature. A higher toner T.sub.g translates into a higher blocking temperature. Due to a T.sub.g to molecular weight dependent relationship for most polymers, toner blocking temperature is determined primarily by the lower molecular weight components of the resin composition. In practice, a blocking temperature of 115.degree. C. is required by toner use and storage considerations. Consequently toner resins with broad molecular weight distributions or polydispersities greater than 3 typically require T.sub.g values greater than 57.degree. C. to satisfactorily pass a toner blocking test at 115.degree. F. (46.1.degree. C.). Monomodal poly(styrene-butadiene) resins with M.sub.n near 20,000 or greater require a T.sub.g of only 51.5.degree. C. to pass the blocking test at 110.degree. F., and 54.degree. C. to pass the blocking test at 115.degree. F. Similarly, Spar II resin, available from Goodyear, with M.sub.w near 8,000 and a T.sub.g of 54.degree. C. obtained by reprecipitation to remove low molecular weight components passes the blocking test at 115.degree. F. With the low molecular weight components present, Spar II fails the blocking test at 110.degree. F.
The gloss properties of fused toner images are dependent on M.sub.w and T.sub.g. Fused toner image gloss increases with decreasing molecular weight because it is believed low molecular weight, low viscosity polymers show increased flow when heated. Gloss at lower fusing temperatures improves with decreasing toner resin T.sub.g for the same reason. Thus, high image gloss at low fuser set temperatures is best achieved with low M.sub.w and low T.sub.g toners. By contrast, improved toner crease test fix level is best achieved with low T.sub.g and high M.sub.w toners. Thus, there is a trade off in toner properties required for matte or glossy images which must be optimized to achieve desired toner performance and multi level gloss images.
Toner resin T.sub.g is the principal determinant in toner MFT as measured by crease test properties. Toner resin M.sub.w is the principal determinant in hot offset temperature, fusing latitude and image gloss characteristics. Toners made with low T.sub.g, high M.sub.w copolymers are preferred for improved fix by crease test and broad fusing latitudes. Low T.sub.g and low M.sub.w copolymers are preferred for forming high gloss images at low fusing temperatures with poor crease test fusing latitude. Low M.sub.w toner resins generally fare worse in crease tests compared with high M.sub.w toner resins. Thus, in embodiments of the present invention high gloss (low M.sub.w) resin and a low gloss (high M.sub.w) resins are required to provide glossy and matte image appearances, respectively, for toners fused under the same conditions. Thus, gloss and gloss fusing latitude are improved by low molecular weight polymers while fusing latitude as determined by crease test methods deteriorates. Toner resins with broad polydispersities as taught in the aforementioned prior art patents attempt to compromise between these conflicting gloss/crease toner properties but they do not represent an optimized solution. In embodiments of the present invention, superior toner materials having optimum crease and gloss performances are obtained by optimizing toner performance using monomodal, monodisperse resins. Monomodal, monodisperse resins represent an excellent compromise between the conflicting performance criteria of crease and gloss.
Polymer structure, T.sub.g and M.sub.w determine the fusing behavior of xerographic toners. Monomodal, monodisperse polymers of the present invention allow molecular weight and T.sub.g contributions to the fusing event to be separated and defined. High molecular weight components in a broad molecular weight distribution resin confer the following properties to a toner: high crease and high gloss minimum fix temperatures, because the T.sub.g and M.sub.w of the high molecular weight component are greater than low molecular weight components; broad crease fusing latitude; low gloss at low fusing temperatures; poor tape transfer test properties; good crease test; good polymer mechanical properties; slow jetting rate; high melt viscosity; non-blocking behavior; large particle toners which are difficult to form by jetting; and toner images with poor projection efficiencies unless very high fusing temperatures are used. The low molecular weight component in a broad molecular weight resin confers the following properties to a toner: low gloss and low crease minimum fix temperature, because T.sub.g and M.sub.w of this component are smaller; poor crease fusing latitude; high gloss at low fusing temperatures; good tape test properties; poor crease test properties; poor mechanical properties; fast jetting rate with the formation of small particle toner; low melt viscosity; poor toner blocking behavior; and good transparency image projection efficiency at low fusing temperatures. The use of monomodal, monodisperse resins of the present invention allows matte or glossy toner properties to be selected and tailored for optimum performance in the aforementioned toner properties and tests.
There is a very narrow window of opportunistic materials design for low melt toner properties and this window is further constrained by image appearance (gloss or matte images), fusing properties (low melt with broad fusing latitude), toner blocking temperature requirements and jetting rates. Furthermore, toner resin designs must be highly reproducible to ensure consistent low melt toner performance. The preparation of useful low melt toner materials having the aforementioned desirable or preferred properties is consistently achievable with monomodal, monodisperse resins in embodiment of the present invention.
Suitable monomodal polymer resin preparation processes include known radical, anionic, cationic, metathesis and group transfer methodologies. These polymerization processes can be either "living" or "pseudoliving" with reversibly reactive terminating end groups. A reference containing a general discussion of useful methods of polymer synthesis, characterization and evaluation is found in "Macromolecules," 2nd Edition, Vol. 1 and 2, H-G Elias, Plenum, N.Y., 1984, the disclosure of which is incorporated by reference herein in its entirety. Optimized monomodal, monodisperse resins of the present invention show better low crease and high gloss fusing properties compared with their broad molecular weight counterparts.
Anionic copolymer resins representative of preferred monomodal, monodisperse styrene-butadiene copolymer based toner composition characteristics described herein, possess fusing latitudes of between 16.degree. C. (M.sub.w 25,080) and 46.degree. C. (M.sub.w 62,700), compared with 40.degree. C. for a control toner comprised of a copolymer of styrene-n-butyl methacrylate, carbon black and cetyl pyridinium chloride (M.sub.w 45,500). The T.sub.g and M.sub.w of anionic copolymers of the present invention were precisely selected and reproducibly prepared under carefully controlled conditions. Poly(styrene-butadiene) copolymer T.sub.g is highly dependent on butadiene content, molecular weight, and 1,2-vinyl content. At a fixed number of 1,2-vinyl groups, the T.sub.g of random anionic styrene-butadiene copolymers is dependent on butadiene content in the copolymer. Compared with polystyrene, the T.sub.g values of random anionic styrene-butadiene copolymers with 80 and 87 weight percent 1,2-vinyl contents are relatively insensitive to molecular weight see, for example, Example II. Toner blocking temperature is dependent on toner T.sub.g. Minimum fix temperature (MFT) determined using a Xerox 1075.TM. photocopier operated at 11 inches per second by 65 crease metric increases by 1.5.degree. C. for each 1.degree. C. increase in toner T.sub.g. MFT at 65 crease is relatively insensitive to anionic copolymer M.sub.w, and decreases by 0.2.degree. C. for each 1,000 increase in copolymer M.sub.w, for the anionic poly(styrene-butadiene) materials considered. Hot offset temperature (HOT) increases by 0.64.degree. C. for each 1,000 increase in copolymer M.sub.w. Fusing latitude, that is the difference between HOT and MFT, increases with increased copolymer M.sub.w and is relatively independent of copolymer T.sub.g.
Blends of 10 weight percent high M.sub.n (80,000) and 90 weight percent low M.sub.n (20,000) copolymers with comparable T.sub.g values were unsuccessful combinations for enhancing toner fusing latitude. Moreover, a high M.sub.w resin component decreases toner image gloss more than that of a pure low M.sub.w component toner.
For the preparation of matte finish toner resins with broad fusing latitudes the use of silane coupling agents to couple anionic intermediate polymer resins is more effective than blending to increase copolymer M.sub.w. Toners with matte finish have been described in copending U.S. Ser. No. 07/843,051 filed Feb. 28, 1992, the entire disclosure of which is incorporated herein by reference.
Although T.sub.g and M.sub.w of the resin or resin blend determine the fusing behavior of toner, these dependencies change with different polymer classes and structures. Toners made with random anionic styrene-butadiene copolymers with high 1,2-vinyl content fuse at lower temperatures than toner resins made with suspension process styrene-1,4-butadiene copolymers and styrene-n-butyl methacrylate copolymers having comparable T.sub.g values.
In a patentability search there is mentioned various patents, the disclosures of which are incorporated by reference in their entirety:
U.S. Pat. No. 4,973,538 to Suzuki et al., issued Nov. 27, 1990, assigned to Fuji Xerox, discloses a toner containing a colorant and a binder resin formed of a mixture of a low molecular weight component and a high molecular weight component. Both the low molecular weight component and high molecular weight component are made from a styrene-acrylic copolymer, wherein the weight ratio (A) of styrene to acrylic monomer in the low molecular weight polymer component and a weight ratio (B) of styrene to acrylic monomer in the high molecular weight polymer component have a relationship of A/B&gt;1.3. A weight average molecular weight (M.sub.w) and a number average molecular weight (M.sub.n) of the low molecular weight polymer component have a ratio (M.sub.w /M.sub.n) of less than or equal to 3.0. See Col. 4, lines 24-43. The low molecular weight controls lower temperature fixability and the high molecular weight controls anti-offsetting properties. See Col. 3, lines 35-41.
U.S. Pat. No. 4,795,689 to Matsubara et al., issued Jan. 3, 1989, discloses a toner comprising: (1) a nonlinear polymer; (2) a low-melting polymer; (3) a copolymer composed of a segment polymer which is at least compatible with the above nonlinear polymer and a segment polymer which is at least compatible with the low-melting polymer; and (4) a coloring agent. The toner has: (1) a fixability at a low temperature; (2) an anti-offset property; (3) an anti-blocking property; (4) the ability to obtain a color-tone image; (5) the ability to obtain a fogless, clear image; and (6) the ability to obtain a number of repeated copies of an image. The low melting polymer has a number average molecular weight M.sub.n of from 1,000 to 20,000 and a weight average molecular weight M.sub.w of from 2,000 to 100,000. A gel permeation chromatograph is used to measure the weight average molecular weight and the number average molecular weight.
U.S. Pat. No. 4,386,147 to Seimiya et al., issued May 31, 1983, discloses toners containing a colorant and a resin wherein the resin contains at least one particular type of polymer which has a M.sub.w /M.sub.n ratio of about 3.5 to 40. The resin composition comprises a uniform mixture of a low polymer and a high polymer. See Col. 3, lines 11-35.
U.S. Pat. No. 4,910,114 to Hosino et al., issued Mar. 20, 1990, discloses a toner containing a polymer binder comprising a high molecular weight monofunctional monomer which exhibits good flow properties in a molten state, and improved fixing properties in a heat roll fixing process. The high molecular weight monofunctional monomer is obtained by a reaction of a polymer with a compound having an ethylenic double bond and a functional group. The toner characteristics are improved by regulating the polymerization so that the polymer may have a ratio M.sub.w /M.sub.n of 2 to 30.
U.S. Pat. No. 4,968,574 to Morita et al., issued Nov. 6, 1990, discloses toners having a main resin component comprising a lower molecular weight polymer and a higher molecular weight polymer. The lower molecular weight has a weight average molecular weight of 50,000 or less and the higher molecular weight polymer has a weight average molecular weight of 80,000 or more. A toner resin (M.sub.w /M.sub.n) ratio may be adjusted by selecting and compounding ingredients type, composition, molecular weight, and mixing ratio. The ratio may be 3.5 or over. See Col. 3, line 59-Col. 4, line 14.
U.S. Pat. No. 5,001,031 to Yamamoto et al., issued Mar. 19, 1991, discloses an electrophotographic toner composition comprising a vinyl polymer having a number average molecular weight of 1,000 to 10,000, a weight average molecular weight/number average molecular weight ratio of 41-200, and a glass transition temperature of 50.degree.-70.degree. C. These may be controlled to improve a paper-surface smoothening property and low temperature fixing property, balancing an offsetting resistance at high temperature, blocking resistance, and grindability.
U.S. Pat. No. 4,917,984 to Saito, issued Apr. 17, 1990, discloses a toner composition comprising a binder resin and a dye or pigment, wherein the binder resin is a polymer synthesized from a vinylic monomer. The toner has a low fixation temperature, good offset resistance, and excellent background resistance which is provided by using mixtures of the polymer wherein its molecular weight distribution is measured by gel permeation chromatography. See Col. 2, lines 20-46.
U.S. Pat. No. 5,057,392 to McCabe et al., issued Oct. 15, 1991, discloses a low fusing temperature toner powder comprising a polyblend of a crystalline polyester and an amorphous polyester which are crosslinked with an epoxy novolac resin. The crystalline polymer melts at a relatively low temperature and has a relatively low glass transition temperature, while the amorphous polymer has a high glass transition temperature. The crystalline polyester has a number average molecular weight in the range of about 1000 to about 3000 and a weight average molecular weight in the range of about 2000 to about 6000. The amorphous polyester has a number average molecular weight in the range of about 1000 to about 3000 and a weight average molecular weight in the range of about 2000 to about 9000.
Documents disclosing toner compositions with charge control additives include U.S. Pat. Nos. 3,944,493; 4,007,293; 4,079,014; 4,394,430; and 4,560,635 which illustrates a toner with a distearyl dimethyl ammonium methyl sulfate charge additive. These toners are prepared, for example, by the usual known jetting, micronization, and classification processes. Toners obtained with these processes generally possess a toner volume average diameter of form between about 10 to about 20 microns and are obtained in yields of from about 85 percent to about 98 percent by weight of starting materials without classification procedure.
There is a need for black or colored toners wherein the aforementioned properties are controllable and preferably selectable. There is also a need for black and colored toners that are non-blocking, such as from about 115.degree. F. to about 120.degree. F., of excellent image resolution, non-smearing and of excellent triboelectric charging characteristics. In addition, there is a need for black or colored toners with low fusing temperature, of from about 110.degree. C. to about 150.degree. C., of high or selectable gloss properties such as from about 50 gloss units to about 85 gloss units, of high projection efficiency, such as from about 75 percent efficiency to about 95 percent efficiency or more, and in addition result in developed images with minimal or no paper curl or fuser roller hot offset.